1. Field of the Invention
The present invention relates to power semiconductor devices, especially to Insulated Gate Bipolar Transistor (IGBT).
2. Related Background Art
Insulated Gate Bipolar Transistor (IGBT) is the preferred semiconductor device for high power applications, capable of providing high blocking voltage and high conducting current and can be controlled conveniently. There are many different IGBT structure designs. A dummy cell structure is often used in those IGBT design, as disclosed in U.S. Pat. No. 8,633,510 (which issued to Matsura et al. on Jan. 21, 2014) which is incorporated herein by reference and is hereinafter referred to as the '510 patent.
A typical IGBT structure described in the '510 patent is shown in FIG. 1. This structure includes: a metal collector layer 12 on backside, P-type collector layer 11, N-type field stop layer 10 and N− drift layer 9. On top of the device, there are active cells and dummy cells. The active cell and dummy cell are separated by a gate trench. The trench structures are formed by polysilicon 3 and gate oxide layer 7. Polysilicon 3 is connected to gate electrode. There are N+ region 1 and P+ region 2 in active cells, and they are connected to metal emitter layer 5 through the window in the insulation layer 4. The P-well region 6 in active cells is connected to emitter electrode through P+ region 2. In the dummy cells, there are deep P-wells 8. P-well 8 is not connected to any electrode, and therefore is electrically floating.
The characteristics of FIG. 1 structure is the deep P-well 8 used in the dummy cells and has the advantage of reduced gate capacitance. However, this deep P-well design requires additional diffusion process and thus higher process cost. This deep P-well 8 also forms a large area PN junction with N− drift layer 9, reducing stored carrier density in forward conduction mode and thus the forward saturation voltage (Vcesat) is relatively high.
Accordingly, what is needed is a new design of IGBT structure with reduced process cost and lower Vcesat.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings